Which nerve is primarily responsible for motor function in the face?

The facial nerve is primarily responsible for motor function in the face because it innervates the muscles of facial expression. This nerve plays a crucial role in controlling movements such as smiling, frowning, and other expressions. It emerges from the brainstem and travels through the temporal bone, branching out to various parts of the face.

In contrast, the trigeminal nerve is mainly responsible for sensory functions in the face, including sensations of touch, pain, and temperature. It also has a minor motor component that innervates the muscles used for chewing, but its primary role is not motor expression.

The vagus nerve primarily governs functions related to internal organs, particularly in the parasympathetic nervous system, while the accessory nerve is involved in shoulder movement and primarily innervates muscles like the sternocleidomastoid and trapezius. Neither of these nerves has a significant role in facial motor functions.

Therefore, the facial nerve is the key nerve for motor functions related to facial expression, which makes it the correct choice in this question.